1. Field of the Invention
This invention relates to a temperature-sensitive fluid fan coupling which controls the rotation of a fan for cooling an automobile engine to ensure that the supply of cooling air to the engine be controlled in accordance with the mode of its operation.
2. Description of the Prior Art
A known fan coupling of the type to which this invention pertains is constructed as shown in, for example, FIG. 35 of the accompanying drawings. It includes a driving disc 27 secured to a rotary shaft, having a thick portion 27-1 along its outer edge, and enclosed in a torque transfer chamber 24 in such a way that a torque transfer gap may be left between the disc 27 and the inner wall surface of a closed housing defining the torque transfer chamber 24. A partition 25 divides the interior of the closed housing into the torque transfer chamber 24 and an oil reservoir 26.
The thick portion 27-1 of the driving disc 27 is intended to provide an improved force for torque transfer and an improved pumping action by a dam not shown. The oil in the torque transfer chamber 24, however, causes the "accompanying rotation" of the fan as shown by a curve .beta. in FIG. 31 or 32, when the engine is restarted from its stop position, or is rapidly accelerated from its low to its high input operation during the travel of the vehicle. The "accompanying rotation" of the fan means a sharp increase of its rotation, which produces an abnormally large noise, and also disables the warming up of the engine in the cold season.
The amount of oil in the torque transfer chamber is equal to the amount of oil which is supplied from the oil reservoir to the torque transfer chamber through a flow control hole made in the partition, minus the amount of oil which is collected from the torque transfer chamber by the dam. The fan, however, continues rotating without lowering its speed for some time after the flow control hole is closed as a result of a drop in ambient temperature to discontinue the supply of oil from the oil reservoir to the torque transfer chamber. This is due to the fact that the dam has so small a power of collecting oil from the torque transfer chamber that there is a time lag before the amount of oil in the torque transfer chamber is so reduced that the transfer of torque is discontinued.
The oil collecting power of the dam increases in proportion to the difference between the speeds of input and output rotation (or the relative speed of rotation). If the speed of input rotation is low, therefore, the power is relatively small, particularly during the beginning of operation at a low temperature. Thus, the fan keeps a high rotating speed owing to its "accompanying rotation" as shown at .beta. in FIG. 31, and there occurs a large hysteresis or lag in the operation of the dam due to a delay in temperature elevation or drop. If there is a large hysteresis, the dam has only a small power of collecting oil from the torque transfer chamber, and the oil remaining in the torque transfer chamber causes the "accompanying rotation" of the fan.
The thick portion 27-1 of the driving disc 27 prevents its preparation by pressing, and makes it imperative to rely upon cutting from a casting having a correspondingly large thickness. This process is less efficient and makes the disc, and thereby the whole device, more expensive. Moreover, the thick portion 27-1 adds to the weight of the disc and thereby of the whole device.